The speed of aircrafts is generally derived from the difference between a total or dynamic pressure and a static pressure of the aircraft's surrounding air. As air is a compressible fluid, velocity is determined with a relatively complex relation, including the square root of an expression including the difference between the total or dynamic pressure and the static pressure.
The static pressure is generally sensed by a tube length substantially parallel to the flow direction and having a wall with small drilled holes. The total pressure is measured with a Pitot probe, including a tube which has an internal diameter of some millimeters and ends at an orifice at the impact point of a fluid on the tube.
It has been found that the measured total pressure has a value which presents only small variations when the tube length constituting the Pitot probe is tilted relative to the flow velocity vector in a relatively small angle range of about .+-.20.degree..
On the contrary, it has been found that the measured static pressure is sometimes greatly erroneous. In fact, when a tube having by example eight small holes at its perimeter is tilted relative to the velocity vector of the flow in which it is disposed, some holes are more exposed than other ones to the fluid flow and transmit a part of the total pressure while other holes are in a slight depression. It has been found that, in the case of an aircraft, especially during take-off and landing, errors on static pressure can approach or exceed 100 mbar. These errors are especially detrimental because they are integrally incorporated in the pressure difference.
It is known that speed indications have a paramount importance during certain operations such as take-off and landings when the static pressure tapping tube is not in its nominal position.